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Dielectric and impedance spectroscopic studies of lead-free barium‐calcium‐zirconium‐titanium oxide ceramics
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Dielectric and impedance spectroscopic studies of lead-free barium-calcium-zirconium-titanium oxide ceramics Muhammad Asif Rafiq, Muhammad Nadeem Rafiq, K Venkata Saravanan
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Cite this article as: Muhammad Asif Rafiq, Muhammad Nadeem Rafiq, K Venkata Saravanan, Dielectric and impedance spectroscopic studies of lead-free barium-calciumzirconium-titanium oxide ceramics, Ceramics International, http://dx.doi.org/10.1016/j. ceramint.2015.05.107 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dielectric and impedance spectroscopic studies of lead-free barium-calcium-zirconium-titanium oxide ceramics Muhammad Asif Rafiqa, ‡, Muhammad Nadeem Rafiqc,d, and Venkata Saravanan Kb,†,∗ a
Department of Metallurgical and Materials Engineering, University of Engineering and Technology, Lahore, Pakistan b
Department of Materials & Ceramic Engineering/CICECO, University of Aveiro, 3810-193 Aveiro, Portugal c
Electrical& Computer Engineering Department, NDSU, Fargo, USA
d
Department of Electrical Engineering, COMSATS Institute of Information Technology, Lahore, Pakistan
Abstract: Dielectric properties of perovskite structured (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3; [BCZT], ferroelectric ceramics prepared by the conventional solid-state reaction method were investigated by AC impedance spectroscopy. To obtain high density samples, the pressed pellets were sintered at 1450 oC and 1500 oC for 4h. Polarization – Electric field (P-E) measurements of the ceramic samples sintered at 1500 °C showed higher remnant polarization (Pr=12.20 µC/cm2) and coercive field (Ec=4.50 kV/cm) values when compared to Pr=8.02 µC/cm2 and Ec=3.80 kV/cm respectively for the samples sintered at 1450 °C. In addition, BCZT sintered at 1500 °C showed higher dielectric constant as compared to the one sintered at 1450 °C. However, the dielectric † Current affiliation: Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur - 610101, India. ∗ Corresponding author: [email protected] ‡ [email protected]
constant measured as a function of frequency for both the sintered samples showed single maximum value at ~ 105 °C, which is attributed to the structural phase transition (Curie temperature, TC) from ferroelectric, tetragonal phase to paraelectric, cubic phase. AC impedance analysis over the frequency range of 100 Hz-1 MHz for the ceramic sintered at 1500 °C, showed mainly bulk contribution up to 250 °C while bulk and grain-boundary contributions were present above 250oC. Activation energies for conductivity were found to be strongly frequency dependent. The activation energy values are attributed to the conduction of oxygen vacancies via hopping mechanism.
Key Words: Dielectric properties, Impedance, Perovskite, Grain boundary, Activation energy, hopping mechanism
INTRODUCTION: With the increasing environmental awareness, a large number of Pb-free perovskite ceramics are currently under investigation to be used for sensor and actuator applications [1-5]. Barium titanate (BaTiO3) was developed as a piezoelectric actuator ceramic and is one of the most widely used ferroelectric materials in the electronic industry as capacitor, piezoelectric actuators, electro-luminescent panels, pyroelectric detectors, positive temperature coefficient of resistivity (PTCR) sensors[6-9]. It undergoes a ferroelectric to paraelectric phase transition at around 130 °C, where permittivity (İ )މnormally reaches to a maximum value and this temperature is called the Curie temperature, TC. The other two phase transitions are
orthorhombic / tetragonal (O/T) around, T1 = 5°C, and rhombohedral/orthorhombic (R/O), T2 = 90°C [10]. Liu et al.[11]reported a piezoelectric constant, d33 = 620 pC/N in Ba(Ti0.8Zr0.2)TO3 – (Ba0.7Ca0.3)TiO3 ceramics, which is comparable to the high end lead zirconate titanate (PZT) family [9]. It was further claimed that morphotropic phase boundary (MPB) responsible for extra ordinary properties starts from a temperature called the triple point where the cubic paraelectric phase (C), ferroelectric rhombohedral (R) and tetragonal (T) phases coexist. Substitution of A- and/or B- cation sites has been used to tailor the electrical properties of perovskites including BaTiO3 based materials [10-12]. Isovalent dopants are commonly used to alter TC, however it affects the lower temperature phase transition temperatures as well. This process sometimes leads to diffuse phase transition-type behavior. In BaTiO3, Ca+2 - doping has shown a very little effect on TC up to ~ 10 at. %, (Ba0.90Ca0.10TiO3)but causes a dramatic decrease in both O/T and R/O phase transition (TO/T and TR/O) temperatures[10]. Morphotropic phase boundary between rhombohedral and tetragonal phase has been reported for (Ba1-xCax) (Ti0.9Zr0.1)O3, where x ≥ 0.10. Significant increase in İމmax was observed by the introduction of isovalent cations on B site. B-site dopant such as Zr+4 causes a linear decrease in Tc, whereas both TR/O and TO/T increase. The lowering in TC is accompanied by an initial increase in İމmax, but with higher amounts İމmax decreases and permittivity peak becomes increasingly broad. This behavior is termed as ‘‘pinching’’ and is attributed to the coalescence of the three phase transition temperatures, which means that overlap of the three permittivity maxima associated with the individual phase transitions and was observed at a dopant level of around 15 at. % of Zr [10, 12-14]. Liu et al reported that morphotropic phase boundary (MPB) originated from this tricritical triple point of cubic paraelectric phase (C), ferroelectric rhombohedral (R), and tetragonal (T) phases in BCZT system, where permittivity maxima occurs, and also shows extra ordinary electromechanical properties [11].After the report of extra ordinary properties in the composition BaTi0.8Zr0.2O3- 50Ba0.7Ca0.3TiO3 (BCZT), effect of dopant like Pr2O3[15], CuO[16], CeO2 [18], solid solution of BCZT with BiFeO3[17]and their effect on the sintering and properties has been reported[18, 19].Elastic, piezoelectric, dielectric properties, dependence of the piezoelectric coefficient, ferroelectric properties on temperature and piezo-response force microscopic studies of BCZT ceramic has been reported [20, 21]. Complex impedance spectroscopy (CIS) is a powerful non-destructive method, widely used to characterize electrical materials [22, 23].There are four possible ways of analyzing the data, the impedance Z*, the Modulus M*, the Admittance A* or Y *and permittivity İ*. They are interrelated and can be expressed as,
M * = jωC0 Z *
(1)
* = (M * )−1
(2)
Z * = ( A* ) −1
(3)
A* = jω0C0ε *
(4)
where ω is the angular frequency 2 π f, Co is the vacuum capacitance of the measuring cell and electrodes with an air gap in place of the sample, Co = İo/ k, where İo is the permittivity of free space (8.854x 10-14 F/cm), and k = l / A, the cell constant where I is the thickness and A is the area. This analysis displays impedance data in different formalism and provides us the maximum possible information. The display of impedance data in complex plane plot appears in the form of a succession of semicircles due to relaxation phenomena with different time constants of grain (bulk), grain boundary, and interface/polarization in a polycrystalline material. On these bases, it is easy to separate the contribution of various components in the material from overall electrical properties. Systematic study of high temperature dielectric properties and use of complex impedance spectroscopy is still scarce for BCZT ceramics. In this work, we have prepared BCZT by conventional ceramic oxide method and used different calcination temperatures to obtain monophasic material. Sintering temperature of 1450 °C and 1500 °C were used to prepare the samples. XRD analysis, polarization hysteresis loop and dielectric properties as a function of frequency and temperature were done and data was analyzed. Impedance studies of the samples with better properties were conducted i.e. samples sintered at 1500 °C. A set of material constants and properties, which is important for both practical device design and fundamental study, is reported.
EXPERIMENTAL PROCEDURE Polycrystalline ceramic samples of (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 were prepared by the conventional solid state reaction technique. Stoichiometric amount of BaCO3, (Merck, 99%) and TiO2 (Merck, 99%), CaCO3 (Aldrich, 99%), ZrO2 (Aldrich, 99%) were thoroughly mixed in a planetary mill in alcohol medium for 5 h using teflon jars and yttria stabilized zirconia balls. The milled powders were dried, and then calcined at 1200 and 1300°C for 4h in an alumina crucible. The calcined powders were re-milled for 5 h. The pellets were prepared by uniaxially pressing the powders in a 10 mm diameter die at 170 MPa and then isostatically pressed at 200 MPa. The pressed pellets were sintered in air at 1450 °C and 1500 °C for 4 h. The samples were characterized by X-ray diffraction (XRD, Philips X’Pert, Cu KĮ-radiation) for phase identification, to determine phase purity and crystal structure. The diffraction data were refined by the Rietveld method using FullPROF software. To measure electrical properties, the sintered ceramics were polished and silver paste was applied on both sides of the pellets as electrodes and fired at 500oC for 30 mins. The polarization vs. electric field (P-E) relation was measured at room temperature using Aixact TF analyzer 1000 ferroelectric tester at 40Hz. The high temperature dielectric response was assessed as a function of frequency (from 100 Hz to 1 MHz), using a HP 4284A precision LCR Meter. The bulk AC conductivity of the material was evaluated from the complex impedance spectra measured at different temperatures from 30 to 400°C. RESULTS AND DISCUSSION The XRD patterns of the BCZT calcined powders and ceramics sintered at different temperatures are shown in Fig. 1a. As clearly seen from the XRD patterns, the powders calcined at 1350 °C and higher for 2 h yielded a pure, single phase material whereas the powders calcined
at 1200 oC for 2 h show traces of secondary phases. However, the secondary phases diffused into the perovskite lattice during the sintering process, thus yielding a single perovskite phase. Similar results have been reported by Wang et al, where the powders calcined between 1000 ºC and 1300 ºC for 2 h resulted in the formation of secondary phases but disappeared on sintering at 1520 ºC[24].The lattice parameters of tetragonal perovskite phase of the ceramic sintered at 1500 °C calculated after refinement of the XRD pattern by FullProf software was found to be a=4.009 Å and c=4.036 Å with c/a=1.007. These values are very close to the reported values of a=4.007 Å and c=4.025 Å with c/a=1.005 in the literature[11]. The Rietveld refined XRD pattern of the sample sintered at 1500 ºC is shown in Fig. 1b. Fig. 2a and b shows the SEM micrograph and the grain size - frequency histogram of the ceramic samples sintered at 1450 °C and 1500 °C, respectively. As revealed from the histograms in the inset, the sample sintered at 1450 °C shows a narrow grain size distribution with an average grain size of 14.1 µm, while the sample sintered at 1500 °C exhibits a wide distribution of grain sizes with average size of 16.6 µm. Also, the micrographs clearly indicate that the grain size increases with sintering temperature. Fig. 3 shows the ferroelectric hysteresis loops of BCTZ ceramics sintered at 1450°C and 1500 °C, measured at 40 Hz and at room temperature. Though both the samples exhibit fairly well saturated P–E loops, the ceramic samples sintered at 1500 ºC show higher value of remnant polarization (Pr) and coercive field (Ec). This could be attributed to the grain size as its effect dominates the variation of polarization; higher grain size results in an increase of polarization. The Pr and Ec values of the samples sintered at 1500 °C were 12.20 µC/cm2 and 4.50 kV/cm, respectively, as compared to 8.02 µC/cm2 and 3.80 kV/cm of the sample sintered at 1450°C. Also BCZT ceramic samples prepared in the present study which were sintered at 1500°C has
higher remnant polarization values of (Pr =12.20 µC/cm2) when compared to ~ 10 µC/cm2 [20], 9.0 µC/cm2 of [25], 11.69 µC/cm2 [26] and 11.05 µC/cm2 for Pr2O3 doped BCZT [15], and slightly lower Ec=4.50 kV/cm than ~ 5.0 kV/cm[25],but higher than 200 V/mm (2 kV/cm) [20], 1.9 kV/cm [24], 2.2 kV/cm of Pr2O3 doped BCZT [15] . Fig. 4a shows the variation of relative dielectric constant (εr) with temperature (30- 400 o
C) of BCZT ceramics sintered at 1450 °C and 1500 °C, measured at 1 kHz. It is clear from the
figure that the value of εr at room temperature is similar for both the samples, irrespective to the sintering temperature. However, a huge difference in the εr value can be observed at the phase transition temperature (TC) of samples sintered at 1450 oC (εr = 7712) and 1500 oC (εr = 12100). This massive increase/difference in εr with increase in sintering temperature can be attributed to increase in grain size. [26].The increase in grain size with increase in sintering temperature facilitates easier domain wall motion[27] leading to an increase in εr. Moreover, high temperature sintering in air leads to the formation of a strong insulating layer around the grains. These highly insulating grain boundaries have a larger value of resistance and capacitance when compared to that of the grain[28]. The difference in the conductivity of the grain (bulk) and grain boundary results in an increase in the accumulation of surface charge resulting in the increase of interfacial polarization which in turn results in the increase in εr. Fig. 4b and 4c shows the temperature dependence of dielectric constant and loss tangent (tan į) measured at 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz for BCZT ceramics sintered at 1500 ºC. The Curie temperature (TC) was observed ~ 105 °C, which is higher than ~ 95 °C [11], and 85 °C [24]. It is know that grain size has a strong effect on Tc[29], and cell distortion also plays a vital role on phase transitions as reported in BaTiO3[29] and BCZT[24]ceramics. Hence, the change in Tc may be attributed to the different processing conditions and cell volume eơect,
as slightly different lattice parameters of present samples were calculated than the literature. In addition, the values of εrand tan į are higher at lower frequencies and decreases as the frequency increases. The decrease inεr value with frequency can be explained based on the decrease in polarization with the increase in frequency as follows; for a dielectric material the net polarization is given as the sum of contributions from dipolar, electronic, ionic and interfacial polarizations [30]. All the polarizations easily respond to the time varying electric field at low frequencies, but as the frequency of the electric field increases, the contributions of different polarization filters out resulting in the decrease of net polarization that leads to the decrease in the value of εr of the dielectric. The dielectric constant values at room temperature varied between 2465 (1 MHz) and 3410 (100 Hz), which is close to the value 3060 (frequency value not reported) reported in the literature[11]. The above analysis for BCZT ceramics is fairly standard, especially below 200 ºC and has been widely reported in the literature. However, the difference lies in the deviation often noticed in the electrical properties such as, TC, εr, Pr, Ec etc. from one literature report to the other. This deviation is often ascribed to the effect of grain size, porosity, chemical inhomogeneity, especially grain and grain boundary impedance. In order to perform a thorough analysis of the electrical properties and their temperature dependence, it is essential to separate the contribution from grain / bulk and grain boundary components. Hence, the complex impedance spectroscopy (CIS), a non-destructive technique was used in investigating the electrical properties of BCZT ceramics. The analysis of the transformed dielectric data into different formalism is very important to understand the actual picture of the material. The use of function Z* is particularly appropriate for the resistive and/or conductive analysis where the long-range conduction dominates, whereas M* functions are suitable when localized relaxation dominates.
Fig. 5a shows complex impedance spectrum (Nyquist plot, a plot between the real (Z') and imaginary (Z'') of complex impedance Z*) of BCZT ceramic sintered at 1500 °C. The impedance data from room temperature up to about 300 °C is not illustrated in the figure as they just showed a straight line with large slope signifying the high insulating behaviour of BCZT. However, as the temperature increased (especially above TC) the slope decreased and was found to curve towards the major (Z )މaxis forming clear semicircular arcs. The radius of curvature was found to decrease with increasing temperature, which shows the increase in conductivity of the sample with temperature. In general, existence of a single semicircular arc represents the grain interior (bulk) property of the material. However, in the present case, at temperature of 300°C or higher, the spectrum comprises of two semicircular arcs. The two semi-circular arcs in the Nyquist plot are due to contributions from the grain interior (bulk) and grain boundary. The arc at high frequency is ascribed to the grain (bulk) and the one at low frequency to the grain boundary contribution. Fig. 5a shows typical impedance plot, at temperatures between 300 to 400 °C for BCZT ceramics consisting of two semi-circular arcs along with the fitting results of impedance data by using Z-view software (Ver. 3.2c Scribner Associates, Inc). Fitting was done by using an equivalent electric circuit consisting of two resistances in series and capacitive phase element (CPE) in parallel as shown in the inset of Fig. 5a and values of resistance and capacitance for the bulk and grain boundary was calculated. CPE was used for fitting instead of pure capacitor to overcome the dispersion and non-linearties in the values, and expected to show a more accurate representation of the dielectric properties and related data. Fitting the data showed good agreement with the experimental data in all cases. R1 in the circuit represented the bulk resistance, C1 the bulk capacitance, R2 the grain boundary resistance and C2 as grain boundary capacitance. From the obtained values of resistance and capacitance for the bulk and grain boundary, relaxation time for the bulk and grain boundary was calculated by using the relation τ = RC and is listed in table 1. Relaxation time was found to decrease with temperature for the bulk and vice versa was true for grain boundary. The variation of real part of impedance (Z )މwith frequency at various temperatures is shown in Fig. 5b. The magnitude of Z މwas found to decrease with increase in temperature which indicated the increase in ac conduction (ıac) in the sample. This increase in ac conduction with temperature might have appeared due to the contribution of defects like oxygen vacancies in the sample. At elevated temperature, the contribution due to oxygen vacancies is more dominant in perovskite structures. Higher value of Z މat lower frequency and temperature plus merging of Z މat higher frequency (>10 kHz) for all temperatures, clearly
indicates the presence of space charge polarization[31, 32]. At higher temperature the impedance seems to be independent of frequency and temperature. The variation of imaginary part of impedance (Z )ފwith frequency at different temperatures is shown in Fig. 5c. The Zފmax peaks shifts and broadening was observed at higher frequencies with increasing temperature. The appearance of temperature dependent peaks (Zފmax) at a characteristic frequency (Ȧmax=2ʌfmax) can be related to the type and strength of the electrical relaxation phenomena in the material. This peak shift and broadening behavior shows the presence of relaxation process, which is temperature dependent [33]. These relaxation processes may be due to the presence of immobile species at low temperature and defects at high temperature. The shift in the peak position with temperature shows a temperature dependent relaxation process. A clear indication of the dispersion of the resultant curves in lower frequency region at different temperatures was observed. Further, the curves were found to merge at a specific frequency in the higher frequency region. This is due to the reduction in space charge polarization at higher frequency. In perovskite system the major mode of charge transport is a multiple hopping process. This hopping process normally takes place across the potential barriers set up by the lattice structure and the local environment of other atoms / ions. The impedance data have been re-plotted in the modulus formalism as shown in Fig. 6 at the same temperatures as has been shown for the complex impedance plots. Fig. 6a shows two semicircular arcs in the complex modulus plots with a small semicircle at high frequency and a large semi-circular arc in the low frequency region at all the temperatures. The modulus spectrum shows a marked change in the shape with use in temperature suggesting a probable change in the capacitance value of BCZT material as a function of temperature.
Fig. 6b shows the variation of real part of electric modulus (M )މwith frequency at higher temperatures (between 300 °C to 400 °C ). It is characterized by a low value of M މin the low frequency region followed by a continuous dispersion with increase in frequency. It was found that M މvalues saturated to a maximum value in the high frequency region for all temperatures. This sigmoidal increase in the value of M މapproaches to maximum value with increasing frequency, is probably due to the short range mobility of carriers like ions. Fig. 6c shows the plot of imaginary part (M )ފof dielectric modulus at different temperatures. The peak was found to shift towards higher frequency side with increasing temperature. Physically, the peak in the imaginary part of the electric modulus defines the regions where the carrier can move at long distances (left to the peak) or confinement (right to the) peak. Also, a peak in the M ފimaginary part indicates a dielectric relaxation process in the solid, and the frequency to the maximum indicates the mean relaxation time of this process. As can be seen, the imaginary part of the electric modulus exhibits a very well defined peak. In our case the peak is relatively symmetrical in a considerable frequency range at each measured temperature. Fig. 7 shows the variation of relaxation time with inverse of temperature. The relaxation time IJ was calculated from the fitting of Nyquist plot using the relation, IJ = RC where R is the resistance and C is the capacitance. The plot of IJ versus 1000/T gives a straight line which can be approximated to the relation, τ=τ0exp(Ea/KBT) [34], where IJ0 is the preexponential factor, Ea is the activation energy and kB is the Boltzmann constant. The calculated activation energy of BCZT bulk and grain boundary is0.69 and 1.44eV respectively. The relaxation time is in 105
seconds, which can be attributed to the hopping conduction mechanism[35].
Fig. 8a shows the AC electrical conductivity (σac(ω)) as a function of frequency at different temperatures from 300 °C to 400 °C. The analysis of the AC-conductivity shows that the values of conductivities are in the range of (10-5- 10-3) (S/m) at frequencies of 1-106 Hz for BCZT ceramic. The plots showed a dispersive nature at lower frequency regime which became narrowed at higher frequencies. As it could be observed, the slope change in lower temperature region is more prominent. This may be the indication of maximum ionic motion taken place, which involves localized hopping with either rotation or translation of the mobile species[32].The behavior of the AC conductivity with temperature for BCZT ceramics is shown in Fig. 8b. The graphs can be divided, independent of frequency, in four regions characterized by different slopes, as shown in the figure. According to this, one ferroelectric region designated by FE I and three paraelectric regions PE I, PE II, and PE III are marked out with dashed lines. Every region is characterized by different slopes indicating the presence of different conduction mechanisms associated with their corresponding values of activation energy (Ea). The AC conductivity increases with measuring frequencies especially at low temperatures. As the temperature increases, the conductivity becomes frequency independent with a slight increase on the higher frequency side. The values of Ea were calculated assuming an Arrhenius behavior according to the equation; § E · (5) ı ac = ı 0 ¨ − a ¸ © K bT ¹ Where, ı 0 stands for the pre-exponential terma, ıac is the ac conductivity, Ea is the activation
energy, Kb is the Boltzmann constant, and T is the temperature in Kelvin scale. Activation energy (Ea) values are summarized in Table 2 for various frequencies. The slope variations and corresponding changes of activation energy for the AC conductivity observed between FE I and PE I regions can be of order-disorder type and attributed
to the readjustment of the lattice during the transition of crystalline phase going from tetragonal cubic structure at room temperature toward cubic structure near the ferro-paraelectric transition temperature in agreement with the structural reports on the BCZT ceramics[11, 24, 25]. The values of Ea obtained for the FE I and PE region from 0.024 - 0.25 eV and 0.47 - 0.62 eV are in correspondence with the hopping charge mechanisms [36]. In the PE II and PE III, the activation energy varied from 0.80 - 1.10eV and 0.69 - 0.75 eV at different frequencies. For ferroelectric perovskites, values of activation energy for conduction in the range of 0.6 - 1.2 eV suggest that the electrical conduction is due to the motion of the electrons from the second ionization of oxygen vacancies [36-38]. At lower temperatures, the oxygen vacancies exhibit low mobility; however, with the increase in temperature they are activated and contribute to the observed electrical behavior. The obtained activation energy for conduction, which is very close to the values associated with double ionized oxygen vacancies (Vo¨) reported for other ceramic oxides suggests oxygen vacancies hopping mechanism [36]. CONCLUSIONS: (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3; BCZT lead free piezoelectric ceramic has been synthesized by conventional solid state reaction technique. XRD studies showed that monophasic ceramic were formed only at a calcination temperature of 1350 °C. Samples sintered at 1500°C showed higher remnant polarization (Pr=12.20 µC/cm2), coercive field (Ec=4.50 kV) and dielectric constant (εr ~ 12,000) respectively. Detailed studies of dielectric and electrical properties indicate that the material has mainly two contribution of grain (bulk) and grain boundary. The activation energy values suggest that the electrical conduction in BCZT is mainly due to the mobility of the ionized oxygen defects via hopping mechanism.
Acknowledgements KVS acknowledges UGC, Govt. of India for providing the Start-up Grant and other infrastructure facilities and the Foundation for Science and Technology(FCT), Portugal for the grant SFRH/BPD/80742/2011. The authors acknowledge Paula Vilharinho and Elisabaete Costa, Universidade de Aveiro, for extending the infrastructural and laboratory facilities.
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Table Captions: Table 1. Resistance, capacitance values and relaxation time determined for bulk and grain boundary. Table 2. Activation energy values Ea in eV for ac conduction, calculated assuming an Arrhenius behavior for each of the marked regions in Fig. 8 for different frequency values.
Figure Captions:
Fig.1. (a)X-ray diffraction patterns of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics (a) calcined and sintered at various temperatures. (b)X-ray diffraction pattern and the corresponding refinement data of the sample sintered at 1500 ºC. Fig.2. Scanning electron micrographs and the corresponding grain size distribution of BCZT ceramic samples (a) sintered at 1450 ºC and (b) sintered at 1500 ºC. Fig. 3. P-E loops of BCZT ceramics sintered at 1450 °C and 1500 °C. Sample sintered at 1500 °C showed higher remnant polarization and coercive field value. Fig. 4. Dielectric constant measured as a function of temperature of BCZT ceramics a) at a frequency of 1 kHz sintered at 1450°C, (b) at various frequencies from 100 Hz to 1MHz sintered at 1500°C, and dielectric loss tan į at various frequencies from 100 Hz to 1 MHz sintered at 1500°C (c) Dielectric loss for the samples sintered at 1500° C and at various frequencies from 100 Hz to 1 MHz. Fig. 5. Shows the (a) Nyquist plot at different temperatures along with the fitting results using electric circuit consisting of two resistance and one CPE, (b) real part and (c) imaginary part of impedance as a function of frequency of BCZT ceramic samples sintered at 1500oC, measured at various temperatures. Fig. 6. BCZT ceramics (a) M ފvs. M މat different temperatures (b) M މvs. frequency and (c) Mފ vs. frequency from 100 Hz to 1 MHz at various temperatures.
Fig. 7.Variation of relaxation time for the bulk (grain interior) and grain boundary vs. inverse of temperature for BCZT ceramics sintered at 1500°C. Fig.8.Variation of ac conductivity vs. (a) frequency at various temperatures and (b) inverse of temperature at different frequencies for BCZT ceramics sintered at 1500 °C.
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(a)
(b)
Fig. 1. X-ray diffraction patterns of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3ceramics(a) calcined and sintered at various temperatures and (b) represents the refinement of sample sintered at 1500 ºC.
(a)
(b)
Fig. 2.SEM micrographs of ceramics sintered at 1450 and 1500 °C, respectively. The inset shows the grains size distribution.
Fig. 3. P-E loops for the (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics sintered at 1450 °C and 1500 °C. Sample sintered at 1500 °C showed higher remnant polarization and coercive field value.
(a)
(b)
(c)
Fig. 4. Dielectric constant of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics a) at a frequency of 1 kHz sintered at 1450°C, (b) at various frequencies from 100 Hz to 1MHz sintered at 1500°C, and dielectric loss tan δ at various frequencies from 100 Hz to 1 MHz sintered at 1500°C(c) dielectric loss for the ceramic sintered at 1500° C and at various frequencies from 100 Hz to 1 MHz. The ceramic sintered at 1500 °C showed higher dielectric constant and loss value started to increase at high temperature of almost 170 °C.
(a)
R1
R2
CPE1
CPE2
(b)
(c)
Fig. 5. (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics(a) Nyquist plot at different temperatures along with the fitting results using (b) electric circuit consisting of two resistance and CPE (c) real part impedance and d) imaginary part of impedance versus frequency from 100 Hz to 1 MHz at various temperature along with fitting results.
(a)
(b)
(c)
Fig. 6. (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics(a) M; vs. M; at different temperatures along with the fitting results (b) M; vs.frequency and (c) M; vs. frequency from 100 Hz to 1 MHz at various temperatures.
Fig. 7. Variation of relaxation time for the bulk (grain interior) and grain boundary vs. inverse of temperature for (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramicssintered at 1500 °C.
(a)
(b)
Fig. 8. Variation of ac conductivity vs. (a) frequency at various temperatures and (b) inverse of temperature at different frequencies for BCZT ceramics sintered at 1500°C.
Dielectric and impedance spectroscopic studies of lead-free barium-calcium-zirconium-titanium oxide ceramics Muhammad Asif Rafiq, Muhammad Nadeem Rafiq, K Venkata Saravanan
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PII: DOI: Reference:
S0272-8842(15)01051-2 http://dx.doi.org/10.1016/j.ceramint.2015.05.107 CERI10684
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Ceramics International
Received date: Revised date: Accepted date:
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Cite this article as: Muhammad Asif Rafiq, Muhammad Nadeem Rafiq, K Venkata Saravanan, Dielectric and impedance spectroscopic studies of lead-free barium-calciumzirconium-titanium oxide ceramics, Ceramics International, http://dx.doi.org/10.1016/j. ceramint.2015.05.107 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dielectric and impedance spectroscopic studies of lead-free barium-calcium-zirconium-titanium oxide ceramics Muhammad Asif Rafiqa, ‡, Muhammad Nadeem Rafiqc,d, and Venkata Saravanan Kb,†,∗ a
Department of Metallurgical and Materials Engineering, University of Engineering and Technology, Lahore, Pakistan b
Department of Materials & Ceramic Engineering/CICECO, University of Aveiro, 3810-193 Aveiro, Portugal c
Electrical& Computer Engineering Department, NDSU, Fargo, USA
d
Department of Electrical Engineering, COMSATS Institute of Information Technology, Lahore, Pakistan
Abstract: Dielectric properties of perovskite structured (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3; [BCZT], ferroelectric ceramics prepared by the conventional solid-state reaction method were investigated by AC impedance spectroscopy. To obtain high density samples, the pressed pellets were sintered at 1450 oC and 1500 oC for 4h. Polarization – Electric field (P-E) measurements of the ceramic samples sintered at 1500 °C showed higher remnant polarization (Pr=12.20 µC/cm2) and coercive field (Ec=4.50 kV/cm) values when compared to Pr=8.02 µC/cm2 and Ec=3.80 kV/cm respectively for the samples sintered at 1450 °C. In addition, BCZT sintered at 1500 °C showed higher dielectric constant as compared to the one sintered at 1450 °C. However, the dielectric † Current affiliation: Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur - 610101, India. ∗ Corresponding author: [email protected] ‡ [email protected]
constant measured as a function of frequency for both the sintered samples showed single maximum value at ~ 105 °C, which is attributed to the structural phase transition (Curie temperature, TC) from ferroelectric, tetragonal phase to paraelectric, cubic phase. AC impedance analysis over the frequency range of 100 Hz-1 MHz for the ceramic sintered at 1500 °C, showed mainly bulk contribution up to 250 °C while bulk and grain-boundary contributions were present above 250oC. Activation energies for conductivity were found to be strongly frequency dependent. The activation energy values are attributed to the conduction of oxygen vacancies via hopping mechanism.
Key Words: Dielectric properties, Impedance, Perovskite, Grain boundary, Activation energy, hopping mechanism
INTRODUCTION: With the increasing environmental awareness, a large number of Pb-free perovskite ceramics are currently under investigation to be used for sensor and actuator applications [1-5]. Barium titanate (BaTiO3) was developed as a piezoelectric actuator ceramic and is one of the most widely used ferroelectric materials in the electronic industry as capacitor, piezoelectric actuators, electro-luminescent panels, pyroelectric detectors, positive temperature coefficient of resistivity (PTCR) sensors[6-9]. It undergoes a ferroelectric to paraelectric phase transition at around 130 °C, where permittivity (İ )މnormally reaches to a maximum value and this temperature is called the Curie temperature, TC. The other two phase transitions are
orthorhombic / tetragonal (O/T) around, T1 = 5°C, and rhombohedral/orthorhombic (R/O), T2 = 90°C [10]. Liu et al.[11]reported a piezoelectric constant, d33 = 620 pC/N in Ba(Ti0.8Zr0.2)TO3 – (Ba0.7Ca0.3)TiO3 ceramics, which is comparable to the high end lead zirconate titanate (PZT) family [9]. It was further claimed that morphotropic phase boundary (MPB) responsible for extra ordinary properties starts from a temperature called the triple point where the cubic paraelectric phase (C), ferroelectric rhombohedral (R) and tetragonal (T) phases coexist. Substitution of A- and/or B- cation sites has been used to tailor the electrical properties of perovskites including BaTiO3 based materials [10-12]. Isovalent dopants are commonly used to alter TC, however it affects the lower temperature phase transition temperatures as well. This process sometimes leads to diffuse phase transition-type behavior. In BaTiO3, Ca+2 - doping has shown a very little effect on TC up to ~ 10 at. %, (Ba0.90Ca0.10TiO3)but causes a dramatic decrease in both O/T and R/O phase transition (TO/T and TR/O) temperatures[10]. Morphotropic phase boundary between rhombohedral and tetragonal phase has been reported for (Ba1-xCax) (Ti0.9Zr0.1)O3, where x ≥ 0.10. Significant increase in İމmax was observed by the introduction of isovalent cations on B site. B-site dopant such as Zr+4 causes a linear decrease in Tc, whereas both TR/O and TO/T increase. The lowering in TC is accompanied by an initial increase in İމmax, but with higher amounts İމmax decreases and permittivity peak becomes increasingly broad. This behavior is termed as ‘‘pinching’’ and is attributed to the coalescence of the three phase transition temperatures, which means that overlap of the three permittivity maxima associated with the individual phase transitions and was observed at a dopant level of around 15 at. % of Zr [10, 12-14]. Liu et al reported that morphotropic phase boundary (MPB) originated from this tricritical triple point of cubic paraelectric phase (C), ferroelectric rhombohedral (R), and tetragonal (T) phases in BCZT system, where permittivity maxima occurs, and also shows extra ordinary electromechanical properties [11].After the report of extra ordinary properties in the composition BaTi0.8Zr0.2O3- 50Ba0.7Ca0.3TiO3 (BCZT), effect of dopant like Pr2O3[15], CuO[16], CeO2 [18], solid solution of BCZT with BiFeO3[17]and their effect on the sintering and properties has been reported[18, 19].Elastic, piezoelectric, dielectric properties, dependence of the piezoelectric coefficient, ferroelectric properties on temperature and piezo-response force microscopic studies of BCZT ceramic has been reported [20, 21]. Complex impedance spectroscopy (CIS) is a powerful non-destructive method, widely used to characterize electrical materials [22, 23].There are four possible ways of analyzing the data, the impedance Z*, the Modulus M*, the Admittance A* or Y *and permittivity İ*. They are interrelated and can be expressed as,
M * = jωC0 Z *
(1)
* = (M * )−1
(2)
Z * = ( A* ) −1
(3)
A* = jω0C0ε *
(4)
where ω is the angular frequency 2 π f, Co is the vacuum capacitance of the measuring cell and electrodes with an air gap in place of the sample, Co = İo/ k, where İo is the permittivity of free space (8.854x 10-14 F/cm), and k = l / A, the cell constant where I is the thickness and A is the area. This analysis displays impedance data in different formalism and provides us the maximum possible information. The display of impedance data in complex plane plot appears in the form of a succession of semicircles due to relaxation phenomena with different time constants of grain (bulk), grain boundary, and interface/polarization in a polycrystalline material. On these bases, it is easy to separate the contribution of various components in the material from overall electrical properties. Systematic study of high temperature dielectric properties and use of complex impedance spectroscopy is still scarce for BCZT ceramics. In this work, we have prepared BCZT by conventional ceramic oxide method and used different calcination temperatures to obtain monophasic material. Sintering temperature of 1450 °C and 1500 °C were used to prepare the samples. XRD analysis, polarization hysteresis loop and dielectric properties as a function of frequency and temperature were done and data was analyzed. Impedance studies of the samples with better properties were conducted i.e. samples sintered at 1500 °C. A set of material constants and properties, which is important for both practical device design and fundamental study, is reported.
EXPERIMENTAL PROCEDURE Polycrystalline ceramic samples of (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 were prepared by the conventional solid state reaction technique. Stoichiometric amount of BaCO3, (Merck, 99%) and TiO2 (Merck, 99%), CaCO3 (Aldrich, 99%), ZrO2 (Aldrich, 99%) were thoroughly mixed in a planetary mill in alcohol medium for 5 h using teflon jars and yttria stabilized zirconia balls. The milled powders were dried, and then calcined at 1200 and 1300°C for 4h in an alumina crucible. The calcined powders were re-milled for 5 h. The pellets were prepared by uniaxially pressing the powders in a 10 mm diameter die at 170 MPa and then isostatically pressed at 200 MPa. The pressed pellets were sintered in air at 1450 °C and 1500 °C for 4 h. The samples were characterized by X-ray diffraction (XRD, Philips X’Pert, Cu KĮ-radiation) for phase identification, to determine phase purity and crystal structure. The diffraction data were refined by the Rietveld method using FullPROF software. To measure electrical properties, the sintered ceramics were polished and silver paste was applied on both sides of the pellets as electrodes and fired at 500oC for 30 mins. The polarization vs. electric field (P-E) relation was measured at room temperature using Aixact TF analyzer 1000 ferroelectric tester at 40Hz. The high temperature dielectric response was assessed as a function of frequency (from 100 Hz to 1 MHz), using a HP 4284A precision LCR Meter. The bulk AC conductivity of the material was evaluated from the complex impedance spectra measured at different temperatures from 30 to 400°C. RESULTS AND DISCUSSION The XRD patterns of the BCZT calcined powders and ceramics sintered at different temperatures are shown in Fig. 1a. As clearly seen from the XRD patterns, the powders calcined at 1350 °C and higher for 2 h yielded a pure, single phase material whereas the powders calcined
at 1200 oC for 2 h show traces of secondary phases. However, the secondary phases diffused into the perovskite lattice during the sintering process, thus yielding a single perovskite phase. Similar results have been reported by Wang et al, where the powders calcined between 1000 ºC and 1300 ºC for 2 h resulted in the formation of secondary phases but disappeared on sintering at 1520 ºC[24].The lattice parameters of tetragonal perovskite phase of the ceramic sintered at 1500 °C calculated after refinement of the XRD pattern by FullProf software was found to be a=4.009 Å and c=4.036 Å with c/a=1.007. These values are very close to the reported values of a=4.007 Å and c=4.025 Å with c/a=1.005 in the literature[11]. The Rietveld refined XRD pattern of the sample sintered at 1500 ºC is shown in Fig. 1b. Fig. 2a and b shows the SEM micrograph and the grain size - frequency histogram of the ceramic samples sintered at 1450 °C and 1500 °C, respectively. As revealed from the histograms in the inset, the sample sintered at 1450 °C shows a narrow grain size distribution with an average grain size of 14.1 µm, while the sample sintered at 1500 °C exhibits a wide distribution of grain sizes with average size of 16.6 µm. Also, the micrographs clearly indicate that the grain size increases with sintering temperature. Fig. 3 shows the ferroelectric hysteresis loops of BCTZ ceramics sintered at 1450°C and 1500 °C, measured at 40 Hz and at room temperature. Though both the samples exhibit fairly well saturated P–E loops, the ceramic samples sintered at 1500 ºC show higher value of remnant polarization (Pr) and coercive field (Ec). This could be attributed to the grain size as its effect dominates the variation of polarization; higher grain size results in an increase of polarization. The Pr and Ec values of the samples sintered at 1500 °C were 12.20 µC/cm2 and 4.50 kV/cm, respectively, as compared to 8.02 µC/cm2 and 3.80 kV/cm of the sample sintered at 1450°C. Also BCZT ceramic samples prepared in the present study which were sintered at 1500°C has
higher remnant polarization values of (Pr =12.20 µC/cm2) when compared to ~ 10 µC/cm2 [20], 9.0 µC/cm2 of [25], 11.69 µC/cm2 [26] and 11.05 µC/cm2 for Pr2O3 doped BCZT [15], and slightly lower Ec=4.50 kV/cm than ~ 5.0 kV/cm[25],but higher than 200 V/mm (2 kV/cm) [20], 1.9 kV/cm [24], 2.2 kV/cm of Pr2O3 doped BCZT [15] . Fig. 4a shows the variation of relative dielectric constant (εr) with temperature (30- 400 o
C) of BCZT ceramics sintered at 1450 °C and 1500 °C, measured at 1 kHz. It is clear from the
figure that the value of εr at room temperature is similar for both the samples, irrespective to the sintering temperature. However, a huge difference in the εr value can be observed at the phase transition temperature (TC) of samples sintered at 1450 oC (εr = 7712) and 1500 oC (εr = 12100). This massive increase/difference in εr with increase in sintering temperature can be attributed to increase in grain size. [26].The increase in grain size with increase in sintering temperature facilitates easier domain wall motion[27] leading to an increase in εr. Moreover, high temperature sintering in air leads to the formation of a strong insulating layer around the grains. These highly insulating grain boundaries have a larger value of resistance and capacitance when compared to that of the grain[28]. The difference in the conductivity of the grain (bulk) and grain boundary results in an increase in the accumulation of surface charge resulting in the increase of interfacial polarization which in turn results in the increase in εr. Fig. 4b and 4c shows the temperature dependence of dielectric constant and loss tangent (tan į) measured at 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz for BCZT ceramics sintered at 1500 ºC. The Curie temperature (TC) was observed ~ 105 °C, which is higher than ~ 95 °C [11], and 85 °C [24]. It is know that grain size has a strong effect on Tc[29], and cell distortion also plays a vital role on phase transitions as reported in BaTiO3[29] and BCZT[24]ceramics. Hence, the change in Tc may be attributed to the different processing conditions and cell volume eơect,
as slightly different lattice parameters of present samples were calculated than the literature. In addition, the values of εrand tan į are higher at lower frequencies and decreases as the frequency increases. The decrease inεr value with frequency can be explained based on the decrease in polarization with the increase in frequency as follows; for a dielectric material the net polarization is given as the sum of contributions from dipolar, electronic, ionic and interfacial polarizations [30]. All the polarizations easily respond to the time varying electric field at low frequencies, but as the frequency of the electric field increases, the contributions of different polarization filters out resulting in the decrease of net polarization that leads to the decrease in the value of εr of the dielectric. The dielectric constant values at room temperature varied between 2465 (1 MHz) and 3410 (100 Hz), which is close to the value 3060 (frequency value not reported) reported in the literature[11]. The above analysis for BCZT ceramics is fairly standard, especially below 200 ºC and has been widely reported in the literature. However, the difference lies in the deviation often noticed in the electrical properties such as, TC, εr, Pr, Ec etc. from one literature report to the other. This deviation is often ascribed to the effect of grain size, porosity, chemical inhomogeneity, especially grain and grain boundary impedance. In order to perform a thorough analysis of the electrical properties and their temperature dependence, it is essential to separate the contribution from grain / bulk and grain boundary components. Hence, the complex impedance spectroscopy (CIS), a non-destructive technique was used in investigating the electrical properties of BCZT ceramics. The analysis of the transformed dielectric data into different formalism is very important to understand the actual picture of the material. The use of function Z* is particularly appropriate for the resistive and/or conductive analysis where the long-range conduction dominates, whereas M* functions are suitable when localized relaxation dominates.
Fig. 5a shows complex impedance spectrum (Nyquist plot, a plot between the real (Z') and imaginary (Z'') of complex impedance Z*) of BCZT ceramic sintered at 1500 °C. The impedance data from room temperature up to about 300 °C is not illustrated in the figure as they just showed a straight line with large slope signifying the high insulating behaviour of BCZT. However, as the temperature increased (especially above TC) the slope decreased and was found to curve towards the major (Z )މaxis forming clear semicircular arcs. The radius of curvature was found to decrease with increasing temperature, which shows the increase in conductivity of the sample with temperature. In general, existence of a single semicircular arc represents the grain interior (bulk) property of the material. However, in the present case, at temperature of 300°C or higher, the spectrum comprises of two semicircular arcs. The two semi-circular arcs in the Nyquist plot are due to contributions from the grain interior (bulk) and grain boundary. The arc at high frequency is ascribed to the grain (bulk) and the one at low frequency to the grain boundary contribution. Fig. 5a shows typical impedance plot, at temperatures between 300 to 400 °C for BCZT ceramics consisting of two semi-circular arcs along with the fitting results of impedance data by using Z-view software (Ver. 3.2c Scribner Associates, Inc). Fitting was done by using an equivalent electric circuit consisting of two resistances in series and capacitive phase element (CPE) in parallel as shown in the inset of Fig. 5a and values of resistance and capacitance for the bulk and grain boundary was calculated. CPE was used for fitting instead of pure capacitor to overcome the dispersion and non-linearties in the values, and expected to show a more accurate representation of the dielectric properties and related data. Fitting the data showed good agreement with the experimental data in all cases. R1 in the circuit represented the bulk resistance, C1 the bulk capacitance, R2 the grain boundary resistance and C2 as grain boundary capacitance. From the obtained values of resistance and capacitance for the bulk and grain boundary, relaxation time for the bulk and grain boundary was calculated by using the relation τ = RC and is listed in table 1. Relaxation time was found to decrease with temperature for the bulk and vice versa was true for grain boundary. The variation of real part of impedance (Z )މwith frequency at various temperatures is shown in Fig. 5b. The magnitude of Z މwas found to decrease with increase in temperature which indicated the increase in ac conduction (ıac) in the sample. This increase in ac conduction with temperature might have appeared due to the contribution of defects like oxygen vacancies in the sample. At elevated temperature, the contribution due to oxygen vacancies is more dominant in perovskite structures. Higher value of Z މat lower frequency and temperature plus merging of Z މat higher frequency (>10 kHz) for all temperatures, clearly
indicates the presence of space charge polarization[31, 32]. At higher temperature the impedance seems to be independent of frequency and temperature. The variation of imaginary part of impedance (Z )ފwith frequency at different temperatures is shown in Fig. 5c. The Zފmax peaks shifts and broadening was observed at higher frequencies with increasing temperature. The appearance of temperature dependent peaks (Zފmax) at a characteristic frequency (Ȧmax=2ʌfmax) can be related to the type and strength of the electrical relaxation phenomena in the material. This peak shift and broadening behavior shows the presence of relaxation process, which is temperature dependent [33]. These relaxation processes may be due to the presence of immobile species at low temperature and defects at high temperature. The shift in the peak position with temperature shows a temperature dependent relaxation process. A clear indication of the dispersion of the resultant curves in lower frequency region at different temperatures was observed. Further, the curves were found to merge at a specific frequency in the higher frequency region. This is due to the reduction in space charge polarization at higher frequency. In perovskite system the major mode of charge transport is a multiple hopping process. This hopping process normally takes place across the potential barriers set up by the lattice structure and the local environment of other atoms / ions. The impedance data have been re-plotted in the modulus formalism as shown in Fig. 6 at the same temperatures as has been shown for the complex impedance plots. Fig. 6a shows two semicircular arcs in the complex modulus plots with a small semicircle at high frequency and a large semi-circular arc in the low frequency region at all the temperatures. The modulus spectrum shows a marked change in the shape with use in temperature suggesting a probable change in the capacitance value of BCZT material as a function of temperature.
Fig. 6b shows the variation of real part of electric modulus (M )މwith frequency at higher temperatures (between 300 °C to 400 °C ). It is characterized by a low value of M މin the low frequency region followed by a continuous dispersion with increase in frequency. It was found that M މvalues saturated to a maximum value in the high frequency region for all temperatures. This sigmoidal increase in the value of M މapproaches to maximum value with increasing frequency, is probably due to the short range mobility of carriers like ions. Fig. 6c shows the plot of imaginary part (M )ފof dielectric modulus at different temperatures. The peak was found to shift towards higher frequency side with increasing temperature. Physically, the peak in the imaginary part of the electric modulus defines the regions where the carrier can move at long distances (left to the peak) or confinement (right to the) peak. Also, a peak in the M ފimaginary part indicates a dielectric relaxation process in the solid, and the frequency to the maximum indicates the mean relaxation time of this process. As can be seen, the imaginary part of the electric modulus exhibits a very well defined peak. In our case the peak is relatively symmetrical in a considerable frequency range at each measured temperature. Fig. 7 shows the variation of relaxation time with inverse of temperature. The relaxation time IJ was calculated from the fitting of Nyquist plot using the relation, IJ = RC where R is the resistance and C is the capacitance. The plot of IJ versus 1000/T gives a straight line which can be approximated to the relation, τ=τ0exp(Ea/KBT) [34], where IJ0 is the preexponential factor, Ea is the activation energy and kB is the Boltzmann constant. The calculated activation energy of BCZT bulk and grain boundary is0.69 and 1.44eV respectively. The relaxation time is in 105
seconds, which can be attributed to the hopping conduction mechanism[35].
Fig. 8a shows the AC electrical conductivity (σac(ω)) as a function of frequency at different temperatures from 300 °C to 400 °C. The analysis of the AC-conductivity shows that the values of conductivities are in the range of (10-5- 10-3) (S/m) at frequencies of 1-106 Hz for BCZT ceramic. The plots showed a dispersive nature at lower frequency regime which became narrowed at higher frequencies. As it could be observed, the slope change in lower temperature region is more prominent. This may be the indication of maximum ionic motion taken place, which involves localized hopping with either rotation or translation of the mobile species[32].The behavior of the AC conductivity with temperature for BCZT ceramics is shown in Fig. 8b. The graphs can be divided, independent of frequency, in four regions characterized by different slopes, as shown in the figure. According to this, one ferroelectric region designated by FE I and three paraelectric regions PE I, PE II, and PE III are marked out with dashed lines. Every region is characterized by different slopes indicating the presence of different conduction mechanisms associated with their corresponding values of activation energy (Ea). The AC conductivity increases with measuring frequencies especially at low temperatures. As the temperature increases, the conductivity becomes frequency independent with a slight increase on the higher frequency side. The values of Ea were calculated assuming an Arrhenius behavior according to the equation; § E · (5) ı ac = ı 0 ¨ − a ¸ © K bT ¹ Where, ı 0 stands for the pre-exponential terma, ıac is the ac conductivity, Ea is the activation
energy, Kb is the Boltzmann constant, and T is the temperature in Kelvin scale. Activation energy (Ea) values are summarized in Table 2 for various frequencies. The slope variations and corresponding changes of activation energy for the AC conductivity observed between FE I and PE I regions can be of order-disorder type and attributed
to the readjustment of the lattice during the transition of crystalline phase going from tetragonal cubic structure at room temperature toward cubic structure near the ferro-paraelectric transition temperature in agreement with the structural reports on the BCZT ceramics[11, 24, 25]. The values of Ea obtained for the FE I and PE region from 0.024 - 0.25 eV and 0.47 - 0.62 eV are in correspondence with the hopping charge mechanisms [36]. In the PE II and PE III, the activation energy varied from 0.80 - 1.10eV and 0.69 - 0.75 eV at different frequencies. For ferroelectric perovskites, values of activation energy for conduction in the range of 0.6 - 1.2 eV suggest that the electrical conduction is due to the motion of the electrons from the second ionization of oxygen vacancies [36-38]. At lower temperatures, the oxygen vacancies exhibit low mobility; however, with the increase in temperature they are activated and contribute to the observed electrical behavior. The obtained activation energy for conduction, which is very close to the values associated with double ionized oxygen vacancies (Vo¨) reported for other ceramic oxides suggests oxygen vacancies hopping mechanism [36]. CONCLUSIONS: (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3; BCZT lead free piezoelectric ceramic has been synthesized by conventional solid state reaction technique. XRD studies showed that monophasic ceramic were formed only at a calcination temperature of 1350 °C. Samples sintered at 1500°C showed higher remnant polarization (Pr=12.20 µC/cm2), coercive field (Ec=4.50 kV) and dielectric constant (εr ~ 12,000) respectively. Detailed studies of dielectric and electrical properties indicate that the material has mainly two contribution of grain (bulk) and grain boundary. The activation energy values suggest that the electrical conduction in BCZT is mainly due to the mobility of the ionized oxygen defects via hopping mechanism.
Acknowledgements KVS acknowledges UGC, Govt. of India for providing the Start-up Grant and other infrastructure facilities and the Foundation for Science and Technology(FCT), Portugal for the grant SFRH/BPD/80742/2011. The authors acknowledge Paula Vilharinho and Elisabaete Costa, Universidade de Aveiro, for extending the infrastructural and laboratory facilities.
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Table Captions: Table 1. Resistance, capacitance values and relaxation time determined for bulk and grain boundary. Table 2. Activation energy values Ea in eV for ac conduction, calculated assuming an Arrhenius behavior for each of the marked regions in Fig. 8 for different frequency values.
Figure Captions:
Fig.1. (a)X-ray diffraction patterns of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics (a) calcined and sintered at various temperatures. (b)X-ray diffraction pattern and the corresponding refinement data of the sample sintered at 1500 ºC. Fig.2. Scanning electron micrographs and the corresponding grain size distribution of BCZT ceramic samples (a) sintered at 1450 ºC and (b) sintered at 1500 ºC. Fig. 3. P-E loops of BCZT ceramics sintered at 1450 °C and 1500 °C. Sample sintered at 1500 °C showed higher remnant polarization and coercive field value. Fig. 4. Dielectric constant measured as a function of temperature of BCZT ceramics a) at a frequency of 1 kHz sintered at 1450°C, (b) at various frequencies from 100 Hz to 1MHz sintered at 1500°C, and dielectric loss tan į at various frequencies from 100 Hz to 1 MHz sintered at 1500°C (c) Dielectric loss for the samples sintered at 1500° C and at various frequencies from 100 Hz to 1 MHz. Fig. 5. Shows the (a) Nyquist plot at different temperatures along with the fitting results using electric circuit consisting of two resistance and one CPE, (b) real part and (c) imaginary part of impedance as a function of frequency of BCZT ceramic samples sintered at 1500oC, measured at various temperatures. Fig. 6. BCZT ceramics (a) M ފvs. M މat different temperatures (b) M މvs. frequency and (c) Mފ vs. frequency from 100 Hz to 1 MHz at various temperatures.
Fig. 7.Variation of relaxation time for the bulk (grain interior) and grain boundary vs. inverse of temperature for BCZT ceramics sintered at 1500°C. Fig.8.Variation of ac conductivity vs. (a) frequency at various temperatures and (b) inverse of temperature at different frequencies for BCZT ceramics sintered at 1500 °C.
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BC! Activation energy values Ea in eV for ac conduction, calculated assuming an Arrhenius behavior for each of the marked regions in Figure 8 for different frequency values. 4 ., , ,, ,,,
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Figure
(a)
(b)
Fig. 1. X-ray diffraction patterns of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3ceramics(a) calcined and sintered at various temperatures and (b) represents the refinement of sample sintered at 1500 ºC.
(a)
(b)
Fig. 2.SEM micrographs of ceramics sintered at 1450 and 1500 °C, respectively. The inset shows the grains size distribution.
Fig. 3. P-E loops for the (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics sintered at 1450 °C and 1500 °C. Sample sintered at 1500 °C showed higher remnant polarization and coercive field value.
(a)
(b)
(c)
Fig. 4. Dielectric constant of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics a) at a frequency of 1 kHz sintered at 1450°C, (b) at various frequencies from 100 Hz to 1MHz sintered at 1500°C, and dielectric loss tan δ at various frequencies from 100 Hz to 1 MHz sintered at 1500°C(c) dielectric loss for the ceramic sintered at 1500° C and at various frequencies from 100 Hz to 1 MHz. The ceramic sintered at 1500 °C showed higher dielectric constant and loss value started to increase at high temperature of almost 170 °C.
(a)
R1
R2
CPE1
CPE2
(b)
(c)
Fig. 5. (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics(a) Nyquist plot at different temperatures along with the fitting results using (b) electric circuit consisting of two resistance and CPE (c) real part impedance and d) imaginary part of impedance versus frequency from 100 Hz to 1 MHz at various temperature along with fitting results.
(a)
(b)
(c)
Fig. 6. (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics(a) M; vs. M; at different temperatures along with the fitting results (b) M; vs.frequency and (c) M; vs. frequency from 100 Hz to 1 MHz at various temperatures.
Fig. 7. Variation of relaxation time for the bulk (grain interior) and grain boundary vs. inverse of temperature for (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramicssintered at 1500 °C.
(a)
(b)
Fig. 8. Variation of ac conductivity vs. (a) frequency at various temperatures and (b) inverse of temperature at different frequencies for BCZT ceramics sintered at 1500°C.